Defect Detection in Square Billet Using Phase Information of Longitudinal Waves

Abstract

Defect detection is a technique that is essential for improving the quality of cast billets that have the possibility of defects because of remaining inclusions and stress during cooling and extrusion. The authors have proposed a method to nondestructively detect defects of approximately 2 mm in diameter using the time-of-flight (TOF) of longitudinal ultrasonic waves. This method detects defects as decreases in the apparent sound velocity, which is reconstructed with the aid of computerized tomography (CT), by measuring the TOF with a transmission method. In this study, the method is improved, especially in the measurement of the TOF. Unlike the previous method, which transmits a short-burst chirp signal and calculates the cross correlation between transmitted and received signals, the new method proposed herein transmits a long-burst single frequency signal and measures the TOF from two receiving signals by comparing two measurement planes (one plane is free of pinholes and the other state is unknown). The method proposed here has the disadvantage that the detectable size in the pinhole has an upper limit, but the advantage is that it uses narrowband transducers and enables cheaper calculation cost. Under the assumptions that there is a single pinhole, whose diameter is 2 mm, in a square billet made of duralumin whose size is 100 × 100 (mm2), the proposed method could detect the pinhole successfully.